Add gdisk to prereqs packages

Enable using sgdisk to build the partition table
/x20/teams/spacepark/mendel/kokoro/prod/docker/aiy-board-builder.tar
updated to include the package.

Change-Id: Iebaf3e5efe25c3da14c49af7c622663697ecbe2e
diff --git a/prereqs.mk b/prereqs.mk
index 1b82fac..fb1488f 100644
--- a/prereqs.mk
+++ b/prereqs.mk
@@ -37,6 +37,7 @@
 	device-tree-compiler \
 	dh-python \
 	fakeroot \
+	gdisk \
 	genext2fs \
 	git \
 	gnome-pkg-tools \